Evolution of Atomic Models: Journey from Democritus to Bohr

1. Chapter 13 The Structure of the Atom

2. What do you think atoms are? • What do they look like? Opener - Atoms

3. What is a model? • A detailed, 3D representation of an object, typically on a smaller scale than the original Scientific Models

4. How did Democritus define atoms? • Atoms are not all the same, they are undying, and always moving • All matter is made of indivisible tiny things too small to be seen by the naked eye Democritus – 400 BC

5. How did Aristotle expand his definition? • Matter is made of atoms and will always exist • He defined the four elements as earth, air, water and fire Aristotle – 384 BC

6. Who are alchemists? • Scientists who worked on the philosophers stone or the elixir of immortality • Ancient chemistry which led to advances in modern chemistry and the scientific method Alchemists – 500 BC to 1720 AD

7. How did Lavoisier change the atomic model? • First to name specific elements like oxygen and hydrogen • Developed first periodic table with 33 elements Lavoisier - 1777

8. How did Dalton change it? • Found that elements combine to form compounds • Found that atoms have mass • Started using chemical symbols • Created the billiard ball model Dalton – 1803

9. How did Thompson change it? • By accident he discovered that an atom was made of smaller subatomic particles such as electrons • Developed the plum pudding model Thompson – 1897

10. Thompson’sPlum Pudding Model

11. How did Rutherford change it? • Discovered that atoms had an atomic nucleus • Found that the nucleus was positive and that electrons were negative • Most of an atom is empty space • Designed the gold foil experiment • Developed the planetary model Rutherford – 1898

12. Rutherford’sGold Foil Experiment

13. Particles were shot at a piece of gold foil which hit the nuclei and scattered Rutherford’sGold Foil Experiment

14. Rutherford’sPlanetary Model

15. How did Lewis change it? • Made advances in showing how atoms bond together and how they can share electrons Lewis – 1916

16. How did Bohr change it? • Began the development of the Bohr model • Electrons are located outside the nucleus in specific orbitals • Each orbital has a very specific amount of energy • Electrons need to gain or lose energy to move to a different orbital Bohr – 1922

17. Bohr Model – Carbon Atom

18. Comparison of Atom Models

19. Person who gives out tickets to traffic court. • Half a dime. • What they do with dead people. • What all that glitters is not. • What you do to cattle that get away. • What a horse has to be if he won’t go for water. • Lone Ranger’s horse. • A really “pressing” thing. • What most lectures are. • Twice a half-nium. • Prisoner who sniffed laughing gas. • Soldier from Troy who only fights after dark. • What a doctor should do for his patients (2 available answers). • Roman streaker who wore shoes with wings. • What I do is none of your _______________! • What the police do to drug houses? • How we refer to the guy who had his stomach removed. • What the Lone Ranger did to his horse Opener - Element Puns

20. What is an element? • A substance made up of one type of atom • Each atom MUST have the same number of protons, but may have different numbers of electrons (ions) and neutrons (isotopes) The Atom

21. What sub-atomic particles are found in a nucleus? • Protons • Charge: +1 • Symbol: p+ • Neutrons • Charge: 0 • Symbol: n0 Nucleus

22. What are the properties of electrons? • Found orbiting around the outside of the nucleus • Charge: -1 • Symbol: e- Electrons

23. What is the significance of the atomic number? • Indicates how many protons there are in the element • ONLY way to identify an element Atomic Number

24. Please find the number of protons for: • Hydrogen • Carbon • Silicon • Fluorine • Gold • Potassium • H: 1 • C: 6 • Si: 14 • F: 9 • Au: 79 • K: 19 Atomic Number

25. How do we figure out the mass of an atom? • Use the atomic mass, which is total mass of all of the sub-atomic pieces • Proton – 1 AMU • Neutron – 1 AMU • Electron – 0 AMU (because its so small) Atomic Mass

26. How do we figure out how many neutrons are in a nucleus? • Determine the atomic mass • Subtract the protons (atomic number) • Round DOWN the remaining number (b/c can’t have a part of a neutron) Neutrons

27. Why don’t we subtract out electrons? • We assume that they have no mass because they are so small • Remember, 0 AMU Neutrons

28. Please find the number of neutrons in: • Hydrogen • Carbon • Silicon • Fluorine • Gold • Potassium • H: 0 • C: 6 • Si: 14 • F: 10 • Au: 117 • K: 20 Neutrons

29. Protons& Neutrons

30. How do we determine the number of electrons in atom? • First determine whether or not the element is neutral or charged (ion) • Neutral – same as the atomic number • Charged – add or subtract the charge to/from the atomic number (Mg2+ or O2-) Electrons

31. How do I know whether to add or subtract the charge? • Negative charge means you have more electrons  ADD • Positive charge means you have less electrons  SUBTRACT Remember! Electrons are negative so when you subtract them you are subtracting negatives! Electrons

32. Protons, Neutrons & Electrons – Ions

33. Protons, Neutrons & Electrons – Ions

34. Where do we place electrons around the nucleus? • Bohr’s orbital model shows us that electrons need to be placed in specific orbitals, shells, or energy levels around the nucleus Electron Placement

35. How do you place electrons in a Bohr model? • Determine the total number of electrons in the atom or ion • Place the electrons in circular orbitals following the 2-8-8-2 pattern until you have placed all of them Bohr’s Model of an Atom

36. What is the octet rule? • Atoms “like” to have 8 electrons in the 2nd and 3rd orbitals • 1st orbital is too small to hold 8 electrons • Atoms will react (gain/lose electrons) until they have a FULL outer orbital Bohr’s Model of an Atom

37. What does carbon’s model look like? • Neutral carbon has 6 electrons • Draw a circular diagram with the following: • 1st orbital – 2 • 2nd orbital – 4 (not full) Bohr’s Model of an Atom

38. What are valence electrons? • The electrons in the outermost orbital which determine how an element reacts Valence Electrons

39. Please draw the appropriate Bohr model for: • Li • N • Ne • H • Mg2+ • S2- Bohr’s Model of an Atom

40. Please draw the appropriate Bohr model for: • Cl • Na1+ • Ca • Ar • He0 • H1- • Si4+ Bohr’s Model of an Atom

41. How do Lewis dot diagrams simplify Bohr diagrams? • Dot diagrams only show the outer orbital which holds the valence electrons • Ex. Carbon Lewis Dot Diagrams

42. How do you place the dots (electrons)? • First two go on top • Then fill each side moving clockwise • Draw dots until you run out of electrons Lewis Dot Diagrams

43. Please draw the appropriate Lewis dot diagram for: • N • O • F • H • Be2+ • Cl1- Lewis Dot Diagrams

44. In a Bohr model, what is special about each orbital? • Each orbital represents a specific amount of energy called a quanta • Electrons in an orbital have energy equal to the orbital energy Orbital Energy

45. How do we number orbitals in a Bohr model? • Using the principle quantum number (n) • n = 1 is closest to the nucleus Orbital Energy

46. What is the relationship between orbital number and energy? • Energy is required to keep electrons (-) away from the protons (+) • n = 1 has the lowest energy • As n increases, orbital energy increases • Direct relationship Orbital Energy

47. How do electrons move to higher energy orbitals? • Need to take in energy • Called absorption • Can get energy from: • Light (photons) • Electricity • Heat Moving Electrons

48. Where do electrons move after they absorb energy? • To an empty space in a higher orbital which has a higher energy • Called the excited state Moving Electrons

49. What happens when an electron releases the absorbed energy? • It relaxes and returns to its original orbital called the ground state • As it relaxes, it releases EM energy in the form of IR, visible and UV • Called emission Moving Electrons

50. Absorption vs. Emission